The present invention relates to a method of making fine grained in particular submicron and nano sized cemented carbide powders with low compaction pressure and the powder obtained.
Cemented carbide is made by wet milling of powders forming hard constituents, powders forming binder phase and pressing agents generally PEG (Polyethylene glycol) to a slurry, drying the slurry generally by spray drying, tool pressing the dried powder to bodies of desired shape and finally sintering. During sintering the bodies shrink about 16-20% linearly. The shrinkage depends on pressing pressure, WC grain size, grain size distribution and Cocontent. Pressing tools are expensive to make and are therefore made for a standard linear shrinkage such as 18%. The correct shrinkage is obtained by applying a particular pressing pressure to a certain amount of powder. It is extremely important that the sintered body has a size and shape as close as possible to the desired one in order to avoid expensive post sintering operations such as grinding. However, if the grain size is fine, for example submicron or finer, a higher pressing pressure is needed to obtain a green density that gives the necessary shrinkage. A high pressing pressure is not desirable because of the risk of obtaining pressing cracks in the pressed bodies and an abnormal wear of the compacting tools and even a risk of tool failure and injuries to the operators. Moreover dimensional control of the whole sintered part is facilitated if the pressing pressure is kept within a certain interval.
A method of lowering the compacting pressure for submicron cemented carbide is disclosed in EP-A-1043413. The described method consists in premixing all components except WC for about three hours, adding the WC powder and then finally milling for about ten hours.
A common pressing agent in iron powder metallurgy is Zn-stearate.